In fields such as molecular diagnostics, biosensors are used to test or analyze a sample fluid, such as blood or another body fluid on the presence of one or more target substances. Such target substances include for example an antigen, a micro-organism and/or molecules. To this end, in one type of biosensor, the target substance is bound or captured by a capture substance that is immobilized on a surface within a microfluidic device. The immobilization areas may be called spots. The microfluidic device typically is used for its capability of handling small amounts of sample fluid which are often scarcely available. The presence of the target substance is made tangible via the attachment of a label, such as a fluorescent molecule or any other label which creates a physical effect that can be detected. Optical labels are most commonly used. In order to allow multiplexed sensing, i.e. the sensing of multiple target substances sequentially or simultaneously within a sample fluid with one biosensor, the microfluidic device may comprise multiple capture substances immobilized within one or more spots, either or not organized within an array, at one of its surfaces.
Two arrangements of biosensor devices have been proposed and are used in practice, i.e. a so-called flow-over concept and a so-called flow-through concept.
A biosensor according to the flow-through concept uses a porous membrane having average pore sizes smaller than one micrometer. The spots comprising the capture substances are present on the membranes. By forcing the sample fluid through the membrane, the diffusion distances become very small for the biological molecules comprised in the sample fluid, so that diffusive transport will not limit the adsorption kinetics and efficient capturing of target substance is accomplished.
WO-2007/060580-A1 discloses a microfluidic device comprising a porous membrane enclosed by two housing parts. The membrane is provided with spots of immobilized capture substances for binding target substances. The two housing parts comprise a number of recesses. Together, the recesses of the two housing parts form a channel for guiding a sample fluid. The spots are provided at one or more of the positions where the channel intersects the membrane. The sample fluid that is guided through a particular channel passes each membrane of that channel.